More Clues About Obesity Revealed by Brain-Imaging Study

UPTON, NY — The idea that obese people eat too much because they find
food more palatable than lean people do has gained support from a new
brain-imaging study at the U.S. Department of Energy’s Brookhaven National
Laboratory. The study reveals that the parts of the brain responsible for
sensation in the mouth, lips, and tongue are more active in obese people
than in normal-weight control subjects.

“This
enhanced activity in brain regions involved with sensory processing of
food could make obese people more sensitive to the rewarding properties of
food, and could be one of the reasons they overeat,” said Brookhaven
physician
Gene-Jack Wang, lead author of the study.

Wang acknowledges that obesity is a complex disease with many
contributing factors, including genetics, abnormal eating behavior, lack
of exercise, and cultural influences, as well as cerebral mechanisms,
which are not yet fully understood. In a recent study, he and his team
found that obese people have fewer brain receptors for dopamine, a
neurotransmitter that helps produce feelings of satisfaction and pleasure,
implying that obese people may eat to stimulate their underserved reward
circuits, just as addicts do by taking drugs.

In that study, overall brain metabolism did not differ between obese
and normal-weight controls. But because the sensory appeal of food can be
so important in triggering the urge to eat, Wang and his team wondered
whether obese people might have enhanced metabolic activity in specific
brain regions, particularly those involved in the sensory processing of
food.

To measure regional brain metabolism, the scientists used
positron emission tomography (PET)
after injecting volunteers (10 severely obese and 20 normal controls) with
a radioactively labeled form of glucose, the brain’s metabolic fuel. Known
as FDG, this radiotracer
(invented at Brookhaven) acts like glucose in the brain, concentrating in
regions where metabolic activity is highest. The PET scanner picks up the
radioactive signal to reveal where the FDG is located.

The top images show the brain regions that had
significantly higher metabolic activity in obese subjects compared
with controls (as measured by PET scans and superimposed on an
anatomical MRI image of the brain). The lower images show the same
regions with higher metabolic activity superimposed on a homunculus
-- a diagram of the somatosensory cortex showing which regions are
responsible for sensory input from various parts of the body. The
hot spots lie in the areas receiving sensory input from the lips,
tongue, and mouth. >Download
a hi-res jpeg of this image (530 kB)

The scientists used a computer program to average the PET data from the
subjects within each group, and then compared the obese subjects’ average
with the normal subjects’ result. The program produced three-dimensional
images highlighting areas where the obese group had higher metabolic
activity than the normal-weight group.

The scientists then superimposed these images onto a magnetic resonance
image (MRI) of the whole brain, as well as a diagram of the brain’s
somatosensory cortex, known as a homunculus. A homunculus graphically
illustrates the relative number of sensory nerves innervating various
parts of the body as well as where the input from these nerves is received
on the somatosensory cortex.

The overlapping images revealed “hot spots” — indicating obese
subjects’ higher metabolic activity — in the regions of the parietal
cortex where somatosensory input from the mouth, lips, and tongue is
received. This is also an area involved with taste perception.

“The enhanced activation of these parietal regions in obese subjects is
consistent with an enhanced sensitivity to food palatability, which is
likely to increase the rewarding properties of food,” Wang said.

Taken together with the earlier results on deficient reward circuits,
this enhanced sensitivity could account for the powerful appeal and
significance that food has for obese individuals.

The findings also suggest that pharmacological treatments known to
decrease palatability might be useful along with behavioral therapies in
reducing food intake in obese subjects.

This study will appear in the July 2, 2002, issue of the journal
NeuroReport.

This work was funded by the U.S. Department of Energy, which supports
basic research in a variety of scientific fields, and the National
Institute on Drug Abuse.

The
U.S. Department of Energy's Brookhaven National Laboratory conducts
research in the physical, biomedical, and environmental sciences, as
well as in energy technologies. Brookhaven also builds and operates
major facilities available to university, industrial, and government
scientists. The Laboratory is managed by Brookhaven Science
Associates, a limited liability company founded by Stony Brook
University and Battelle, a nonprofit applied science and technology
organization.